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In our increasingly networked society, quantum cryptography has emerged as a promising alternative to conventional cryptography for ensuring the security of transmitted data. The laws of quantum physics ensure that encryption keys can be exchanged between users in complete secrecy—over short distances, anyway. In optical fiber, though, losses increase with transmission distance, and boosting the signal with optical repeaters would disturb its quantum nature, compromising security.
A research collaboration between the Quantum Technologies group at the University of Geneva, Switzerland, ID Quantique SA, and Corning Inc. has now overcome some of that limitation. The team has demonstrated an optical-fiber-based quantum cryptography scheme that operates over a record 421 km—and at much faster rates than previous demonstrations (Phys. Rev. Lett., doi: 10.1103/PhysRevLett.121.190502).
In the course of the work, the researchers optimized all parts of the quantum key distribution (QKD) system—signal generation, transmission and detection. The effort, they believe, could ultimately enable cheaper and more practical systems that are a commercially viable alternative to conventional technology.
Range limitations
QKD has improved a lot since its first demonstration in 1989, and commercial systems are already available. Current research focuses on extending their range, which is limited by scattering and absorption in optical fibers.
These losses reduce the chance of photons reaching the receiver, and increase exponentially with distance. Conventional optical repeaters cannot regenerate a quantum signal, so as the signal drops off, detector noise dominates and the quantum key that forms the crux of the security protocol can no longer be extracted from the background. Even in the theoretical case of vanishingly low detector noise, there are limits to the stability of the system, and to how long a user will wait to obtain a sufficient key.
Maximizing component efficiency
Increasing the photon pulse rate and maximizing the efficiency of the detector can therefore extend the transmission distance of the QKD system. The University of Geneva-led team started by upping the rate of generation of quantum states to 2.5 GHz using a phase-randomized diode laser. The qubit states were chosen at random using a quantum random number generator.
To transmit the signal, the researchers used spools of ultra-low-loss single-mode optical fiber with a pure silica core and fluorine-doped cladding. The attenuation was about 0.17 dB/km, including connection losses. The team also compensated for chromatic dispersion.
For signal detection, the team used two custom-made molybdenum silicide superconducting nanowire single-photon detectors (SNSPDs), cooled to 0.8 K to reduce noise. Blackbody radiation from the optical fiber also adds noise, and the team filtered that out as well. That step reduced the detectors’ dark-count rate to 0.1 Hz—two orders of magnitude lower than in similar, commercially available detectors.
The final piece of the puzzle was the researchers’ new encoding method for the quantum signals. Their one-decoy state scheme was a modification of a three-state protocol that has already been proven to be secure and was developed specifically to simplify the experimental setup while maintaining efficiency.
A ceiling on transmission distance?
These developments, the authors report, combined to bring an improvement of four orders of magnitude in key receipt rate compared with the rate achieved in the previous, 404-km distance record (which used a different, measurement-device-independent QKD configuration). The researchers ran the system continuously for more than 24 hours, monitoring phase stabilization and temporal alignment with control software, to demonstrate its capability for long-term operation with stable key transmission rates.
Further significant increases in the transmission distance for a fiber-based QKD system such as this may not be feasible. Counteracting the exponential decrease in detected photons with distance would mean exponentially increasing accumulation time, or increasing pulse repetition rate. (One way to get past that potential ceiling would be the development of a “quantum repeater” to handle the role played by optical repeaters in conventional long-haul fiber systems; developing such repeaters is an active area of current research.)
In addition, despite the authors’ improvements, measurement-device-independent QKD is more secure. However, enhancements to the Geneva-led team’s simpler approach could make it the preferred method for particular applications, depending on the relative needs for security, distance and key rate. The results suggest that the scheme could have potential for quantum communication between cities or digital telephony in metropolitan networks.